High resolution time-of-flight measurements

ABSTRACT

This invention relates to apparatus and methods for measuring the time-of-flight of a signal. The signal may be acoustic energy or electromagnetic energy such as x-ray, radio frequency, microwave, millimeter-wave, radar, and laser. Unlike unambiguous ranging devices that measures the phases of two or more signals to determine the time-of-flight and requires long averaging to achieve some degree of accuracy, this invention phase lock one or more transmitter signals to the corresponding received signals in predetermined phase relationships and measures the frequencies of one or more variable frequency oscillators having frequencies several times higher than the frequency of the transmitter signal to determined the time-of-flight with much higher accuracy. An example of an embodiment of this invention is an apparatus that transmits a signal to a receiver and phase lock the transmitted signal to the received signal in a first selected phase relationship. A first frequency of the phase locked signal is determined and a second phase relationship that differs from the first phase relationship by a predetermined fraction of a cycle is selected. The transmitter signal relocks to the receiver signal in the second phase relationship and a second frequency of the relocked signal is determined. The time-of-flight is measured using the first and second frequencies and the predetermined fraction of a cycle difference in phase relationships.

FIELD OF THE INVENTION

The present invention relates to methods and apparatus for measuring the time-of-flight of a signal from a transmitter to a receiver. The methods and apparatus of this invention is applicable to devices that transmit acoustic or electromagnetic energy. Generally it applies to any device that uses the time-of-flight of a signal to make a measurement.

BACKGROUND OF THE INVENTION

Time-of-flight is a method used to measure the time it takes for a signal to travel from a transmitter to a receiver over a distance. The signal used with this method may be acoustic or electromagnetic energy; electromagnetic energy such as x-ray, radio frequency, microwave, millimeter-wave, radar, and laser. Time-of-flight can be used to measure distance, the velocity of a moving object or fluid, or the velocity of a signal over a known distance. It is use in devices such as ultrasonic flow meters, ranging equipments, radar, and navigation systems.

In prior art there are two fundamental methods for measuring time-of-flight; measuring the propagation time of a signal from a transmitter to a receiver (transit time method), and measuring the phase difference between two or more received signals (phase based method).

Ranging devices measure the time-of-flight of a signal from a transmitter to a target and back to a receiver. The time-of-flight and the velocity of the signal through a medium is used to determine the distance to the target.

Ultrasonic transit-time flow meters in prior art measure the time-of-flight of an acoustic pulse transmitted in both direction of fluid flow, and use the difference in the transit time to determine the fluid flow rate. The shape and response time of the pulse is crucial to measuring the time-of-flight with some degree of accuracy. To get good signal to noise ratio a response time between 50-100 pico-seconds and powerful precisely tuned transducers are required; making it relatively expensive. Prior art transit-time devices based on the speed of light require long averaging time and sub-nanosecond timing circuitry to measure distances with good resolution.

Some phase based method use a phase comparator to generate a voltage that is proportional to the phase and use a digital to analog converter to digitize the voltage to determine the phase. Other devices digitize the signal and perform a Fourier transform on the digitized data to determine the phase. The low resolution of analog to digital converters require long averaging time to improve accuracy, and Fourier transform requires intensive computation and increased power usage.

SUMMARY OF THE INVENTION

This present invention relates to apparatus and methods for high resolution time-of-flight measurement at low cost and low power. The object of this invention is accomplished by using at least one variable frequency oscillator to generate the transmitter signal and using the higher frequencies of the oscillator to measure the time-of-flight with much higher accuracy than prior art. One or more oscillators are controlled to phase lock the transmitter signals to the corresponding received signals at different frequencies that differs by a predetermined number of cycles within the time-of-flight, and the time-of-flight is determined based on the frequencies and the difference in the number of cycles.

For improved signal reception and detection the apparatus of this invention comprise sinusoidal signals to which the receiver is tuned to be principally sensitive, noise filtering circuitry, and noise detection circuitry for automatic receiver gain control; providing a high signal to noise ratio with minimum power. Sinusoidal signals can be used to drive transducers over a large range of frequency and bandwidth with no sacrifice in accuracy; thus eliminating the need to use expensive precisely tuned higher frequency transducers as in prior art. This invention does not require expensive sub-nanosecond timing circuitry in devices based on the speed of light as in prior art that rely on accurately detecting the arrival time of a pulse of energy.

An example of an apparatus and methods of this inventions is as follows: A transmitter signal having a predetermined frequency is transmitted to a receiver. The frequency is then controlled to phase lock a transmitter generated signal having no phase delay to the corresponding received signal. The frequency of the phase locked signal is determined to be f₀. The transmitter generated signal is then phase shifted by a fraction of a cycle n_(k) and the transmitter signal relocks to the received signal in the n_(k) phase relationship. The frequency of the relocked signal is determined to be f₁.

The time-of-flight is determined using the following relationships;

N−n _(d0) =t·f ₀;

N−n _(d1) −n _(k) =t·f ₁;

Where N is the unknown integral number of cycles within the unknown time-of-flight t, and n_(d0) and n_(d1) are frequency dependent fraction of a cycle delays due to circuitry, the transmitter, receiver, cables, etc; which can be determined by calibration means.

In many embodiments n_(d0) and n_(d1) are approximately equal (n_(d)) and solving for t and N reduces to:

t=n _(k)/(f ₀ −f ₁);

N=(n _(k) ·f ₀)/(f ₀ −f ₁)+n _(d);

For higher accuracy some embodiments of this invention may first determine N, and in subsequent time-of-flight measurement use N and one frequency measurement (f₀).

To further illustrate the invention, an example of a continuous wave laser ranging apparatus is presented. The apparatus has a system delay of (n_(d)=0.25 cycles). A laser signal (20) is modulated at 10 MHz and transmitted to a target and back to a receiver. The frequency of the signal is controlled to phase lock a transmitter generated signal (21) having no phase delay (25) to a receiver generated signal (22). The signal locks at frequency f₀=10.125 MHz. The transmitter generated signal is delayed by ½ cycle (35) and the signal relocks at frequency f₁=9.375 MHz. (NOTE: at ½ cycle a relock could also occur at 10.875 MHZ, |f₀−f₁|=0.75 MHz).

The time of flight and the range are determined as shown below;

t=n _(k)/(|f ₀ −f ₁|)=0.5/(10,125,000−9,375,000)=0.666666667 usec;

R=t·v/2=(0.666666667 usec)·(300000 km/sec)/2=100 m;

For a time interval of 100 msec and a variable frequency oscillator with a frequency that is 64 times that of the transmitter signal, the counter values are: C₀=64,800,000, C₁=60,000,000;

The range error for counter errors of +1 for one counter and a −1 for the other is determined below:

f ₀=(64,800,000+1)/6.4=10,125,000.156;

f ₁=(60,000,000−1)/6.4=9,374,999.8438;

t=0.5/(10,125,000.156−9,374,999.8438)=0.6666663889 usec;

R=t·v/2=(0.6666663889 usec)·(300000 km/sec)/2=99.999958335 m;

Error: 100 m−99.999958335 m=0.00004 m=0.04 mm;

To reduce the error, the integral number of cycles N can be determined and subsequent time-of-flights determined based on N and f₀ as shown below;

n=(n _(k) ·f ₀)/(f ₀ −f ₁)+n _(d);

n=(0.5−10,125,000.156)/(10,125,000.156−9,374,999.8438)+0.25;

n=6.999;

N=IntegralOf(n+0.5)=7;

A one count error in f₀ would result in;

t=(N−n _(d))/f ₀=(7−0.25)/10,125,000.156=0.6666666564;

R=t·v/2=(0.6666666564 usec)·(300000 km/sec)/2=0.09999999846 km;

Error: (100−99.99999846)m=0.0000015 m=0.0015 mm;

It is to be understood that the above-described embodiments and illustrations are only illustrative of the applications of the principles of the invention and that various modifications may be made by those skilled in the art without departing from the spirit and scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the basic concept of this invention; a phase detector (10) for phase locking the transmitter signal to the receiver signal, and a phase delay circuitry (8).

FIGS. 2 and 3 illustrate transmitter signals phase locked to the receiver signals in different phase locked relationships.

FIGS. 4 and 5 illustrate transmitting and receiving signals in bursts.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1 processor and control electronics (1) provides electrical signals for controlling and processing the transmitter signals, Voltage Control Oscillator (VCO) (2) is selected to generate transmitter signal (6) and a square wave signal (7) indicative of zero-crossing of the transmitter signal. Phase delay circuitry (8) provides selectable predetermined fraction of a cycle phase delays of square wave signal (7) as a first input (9) to Phase Detector (10). A signal generated by reference oscillator (16) is selected as a second input (15) to the Phase Detector for controlling the frequency of the VCO to generate a transmit signal (6) having a predetermined initial frequency. When signal transmission and reception starts, signal arrival is detected by circuitry (13) to start processing the received signal. A receiver generated square wave signal (14) indicative of zero-crossing of the received signal is selected as the second input to the Phase Detector (10) to phase lock the transmitter signal to the receiver signal in the selected predetermined fraction of a cycle phase relationship. The processor monitors status information provided by the control electronics and adjusts the amplitude of the transmitter signal to maintain a preferred receiver signal level. Counter (19) counts the cycles of the VCO for providing periodic counter values to determine the frequency of the VCO. The frequency of the VCO is several times(M) higher than the frequency of the transmitter signal. A sine wave generator 3 is used to generate a drive signal (4) having a frequency to which the transmitter and receiver is most responsive.

FIG. 2 illustrate an Exclusive-OR (XOR) (23) used as a phase delay circuit. One input to the XOR is a transmitter generated square wave signal (26) and the other input is a logic 0 (24) to select a 0° phase delay (25). A phase detector compares the output of the XOR (21) to the zero crossing receiver signal (22) to phase lock the transmitter signal (20) to the received signal in the 0° phase delay relationship.

FIG. 3 illustrate the XOR (33) with a logic 1 input (34) to select a 180° phase delay (35), and the transmitter signal (30) phase locked to the received signal (32) in the 180° phase relationship.

FIG. 4 illustrates alternately transmitting bursts of first (40) and second (41) signals where the first signal is received before transmitting the second signal.

FIG. 5 illustrates alternately transmitting bursts of first (44) and second (45) signals where both signals are transmitted before receiving the first signal. 

What is claimed is:
 1. A method for measuring the time-of-flight of a signal comprising: a. transmitting transmitter signals (6) to a receiver (12), wherein the transmitter signals are generated by one or more high frequency variable frequency oscillators; b. receiving and processing said transmitter signals; c. controlling the frequencies of said frequency oscillators to phase lock the corresponding generated transmitter signal to the corresponding received signal to form at least first and second phase locked signals with frequencies that differ by a predetermined number of cycles within the time of flight; d. making frequency measurements of said variable frequency oscillators for determining the frequency of said phase locked signals; e. using said frequency measurements and said predetermined difference in number of cycles to measure at least one of a set of measurable factors from a group consisting of time-of-flight of said transmitter signal, velocity of said transmitter signal, range of a target, and velocity of said target.
 2. A method for measuring the time-of-flight of a signal as recited in claim lwherein forming said first phase locked signal further comprise transmitting a transmitter signal having a predetermined initial frequency to said receiver then adjusting the frequency to phase lock said transmitter signal to the corresponding receiver signal in a first predetermined phase relationship.
 3. A method for measuring the time-of-flight of a signal as recited in claim lwherein forming said second phase locked signal further comprise transmitting a transmitter signal having the same frequency and phase as said first phase locked signal then adjusting the frequency to phase lock said transmitter signal to the corresponding receiver signal in a second phase relationship that differs from the phase of the first locked signal by a predetermined fraction of a cycle.
 4. A method for measuring the time-of-flight of a signal as recited in claim 1 wherein measuring the time of flight further comprise determining the integral number of cycles within the time of flight of a phase locked signal and using said integral number of cycles and the frequency of the phase locked signal to measure subsequent time of flights with higher accuracy.
 5. Apparatus for measuring the time-of-flight of a signal comprising: a. at least one transmitter (5) disposed to transmit a transmitter signal (6) to a receiver (12); b. at least one receiver (12) disposed for receiving and processing the received signal (11) from said transmitter; c. at least one variable frequency oscillator for generating at least one transmitter signal, wherein the frequency of said variable frequency oscillator further comprise frequencies that are several times higher than the frequency of said transmitter signal; d. means for selecting and applying a drive signal (4) to said transmitter to transmit said transmitter signal to said receiver, wherein said drive signal is provided by said at lease one variable frequency oscillator; e. means for receiving and processing said transmitter signal; f. means for controlling said at least one variable frequency oscillator to phase lock said transmitter signal to the corresponding received signal in predetermined phase relationships to form at least first and second phase locked signals having frequencies that differ by a predetermined number of cycles within the time of flight; g. measurement means to make frequency measurements of said at least one variable frequency oscillator; h. at least one processor circuit and electrical signals (1) configured for processing said transmitter signals, and for measuring said time of flight based on said frequency measurements and said predetermined difference in number of cycles.
 6. Apparatus for measuring the time-of-flight of a signal as recited in claim 5 wherein transmitting a transmitter signal to a receiver further comprise means for transmitting energy onto a target and back to said receiver, wherein said energy is selected from a group consisting of acoustic energy and electromagnetic energy.
 7. Apparatus for measuring the time-of-flight as recited in claim 5 wherein means for forming said first phase locked signal further comprise means for selecting a first phase relationship, a first mode having means for controlling one of said variable frequency oscillator to phase lock said variable frequency oscillator to a master oscillator for generating a first transmitter signal having a predetermined initial frequency; a second mode having means for controlling said variable frequency oscillator to phase lock said first transmitter signal to the corresponding received signal in said first phase relationship; and means for switching between said first and second modes.
 8. Apparatus for measuring the time-of-flight as recited in claim 5 wherein means for forming said second phase locked signal further comprise means for changing the phase relationship of said first phase locked signal by a predetermined fraction of a cycle to relock said first phase locked signal to the corresponding received signal in a second predetermined phase relationship.
 9. Apparatus for measuring the time-of-flight as recited in claim 5 wherein means for forming said second phase locked signal further comprise means for alternately selecting a first and a second of said variable frequency oscillators to transmit said first and second transmitter signals to said receiver; a first mode having means for controlling said first transmitter signal to form said first phase locked signal, a second mode having means for controlling said second variable frequency oscillator to phase lock said second transmitter signal to said first transmitter signal; a third mode having means for controlling said second variable frequency oscillator to phase lock said second transmitter signal to the corresponding received signal in a second phase relationship that differs from the phase of said first phase locked transmitter signal by a predetermined fraction of a cycle, and means for switching modes.
 10. Apparatus for measuring the time-of-flight as recited in claim 5 wherein transmitting a transmitter signal to a receiver further comprise means for generating a carrier signal; means for modulating one or more transmitter signals on top of said carrier signal; means for applying said carrier signal to said transmitter; and means for demodulating said carrier signal by said receiver to produce receiver signals corresponding to said transmitter signals.
 11. Apparatus for measuring the time-of-flight as recited in claim 5 wherein means for determining the frequency of said at least on variable frequency oscillator further comprise at least one counter for counting the cycles of said variable frequency oscillator to provide counter values at periodic intervals to determine said frequency.
 12. Apparatus for measuring the time-of-flight as recited in claim 5 wherein means for phase locking the transmitter signal to the corresponding received signal in a predetermined phase relationship further comprise means for selecting one of said variable frequency oscillators (2) to generate said transmitter signal; means for detecting the arrival of a burst of said transmitter signal by said receiver for processing of said burst; means for generating a first square wave signal (7) indicative of zero-crossing of said transmitter signal; means for generating a second square wave signal (14) indicative of zero-crossing of said received signal; a phase detector (10) for comparing the phase of one of said square wave signal to the shifted phase (8) of the other square wave signal, wherein the phase is shifted by a predetermined fraction of a cycle for providing a control signal to control said selected variable frequency oscillator (2) to phase lock said transmitter signal to the corresponding received signal in said predetermined fraction of a cycle phase relationship; means wherein said control signal is held constant when said selected variable frequency oscillator is unselected or no receiver signal is being processed; and means for varying the response time (17) of said variable frequency oscillator to said control signal as a function of the time of flight and the frequency of said transmitter signal.
 13. Apparatus for measuring the time-of-flight of a signal comprising: a. at least one transmitter (5) disposed to transmit a transmitter signal (6) to a receiver (12); b. at least one receiver (12) disposed for receiving and processing the received signal (11) from said transmitter; c. at least one variable frequency oscillator having a frequency several times higher than the frequency of said transmitter signal for generating said transmitter signal; d. means for receiving and processing said transmitter signal; e. means for transmitting said transmitter signal having a predetermined frequency and controlling the frequency of said transmitter signal to phase lock said transmitter signal to the corresponding received signal to form a phase locked signal; f. measurement means to make frequency measurements of said at least one variable frequency oscillator; g. measurement means to make approximate measurements of the time that it takes for a burst of said transmitter signal to travel from said transmitter to said receiver; h. at least one processor means disposed for determining the integral number of cycles within the time-of-flight of said phase locked signal based on said approximate time measurement and said frequency measurements, and means for using said integral number of cycles and said frequency measurements to make subsequent higher resolution measurement of at least one of a set of measurable factors from a group consisting of time of flight of said transmitter signal, velocity of said transmitter signal, range of a target, and velocity of said target. 